Visual-system lesions in the brains of neonate hamsters result in specific anomalies in the development of optic-tract connections which are correlated with spared or altered function. The clearest results have been obtained in animals with early lesions of the midbrain tectum. Visually elicited turning movements may be spared or altered in specific ways. These functional consequences are correlated with the patterns of anomalous growth of retinotectal axons. We are beginning to understand this growth in terms of certain rules followed by growing axons in the normal brain. Currently, we are analyzing the postnatal development of retinal projections in hamsters. We are varying the age at which lesions are inflicted, in order to define the critical ages at which the damage ceases to cause development of abnormal pathways. In addition, we are using electrophysiological techniques to analyze receptive field properties and topography, of units in thalamus and midbrain which receive abnormal retinal projections after bilateral lesions of superior colliculus at birth. Videotape analysis is used to aid the study of visually guided behavior of hamsters with early brain lesions, and to specify the effects of additional brain surgery, after these animals have reached adulthood. BIBLIOGRAPHIC REFERENCES: So, K.-F. and Schneider, G.E. Abnormal recrossing retinotectal projections after early lesions in Syrian hamsters: a critical-age effect. Anat. Rec., 1976, 184(3), 535-536. Finally, B.L., Schneps, S.E. and Schneider, G.E. Compression of the retinotectal projection after neonatal half-tectum ablation in the hamster. Abstract, Association for Research in Vision and Ophthalmology, 1976.